Copper-tin alloy plating



Patented Nov. 7, 1950 COPPER-TIN ALLOY PLATING Frederick A. Lowenheim, Plainfield, N. J assignor to Metal & Thermit Corporation, New York, N. Y., a corporation of New Jersey No Drawing. Application January 5, 1946, Serial No. 639,409

8 Claims.

This invention contemplates continuous plating with copper-tin alloys and preferably by means of a bath solution which-constitutes the sole source of plating metal, thus avoiding the necessity for using soluble anodes and eliminating the anode control problems which in the past have characterized the use of soluble anodes. Particular objects and advantages will appear hereinafter.

Heretofore the production of satisfactory copper-tin, alloy plate has been complicated by the necessity of using soluble anodes of copper and tin or of copper-tin alloy. Problems of anode .control are multiplied when more than one metal is employed as anode, and several methods have been introduced to overcome these problems. However, all of these methods in their turn have given rise to additional problems. Considerations such as the size and composition of the anodes, their time of immersion in the bath, the maintenance of polarizing films on the tin anodes while avoiding them on the copper anodes, the use of separate circuits with individual control, the tendency to form divalent tin in the bath and so on, have each introduced additional control problems and thereby rendered operation of the bath more and more inflexible.

To avoid the foregoing diiliculties and disadvantages, the problem of simplifying the production of satisfactory copper-tin alloy plate has been approached herein by way of the bath solution, to the end of eliminating the need for soluble anodes and making it practical to rely on the bath itself as the sole source of the alloy deposit.

Almost any plating solution can be operated for a time with insoluble anodes, with the metal content of the bath being kept constant by means of the addition of metallic salts. In the great majority of cases this means of operation is impractical, however, because as the metal plates out the other ion or ions added with the salt continue to build up. For example, in a tin plating bathit is possible to regenerate the solution by continuously adding sodium stannate, but in 7 this event the sodium ion would build up continually and the free caustic content (resulting from the decomposition of the stannate) would gradually rise until the bath composition was completely out of adjustment.

Inthe preferred application of the invention it-"is proposed to use a plating bath containing copper stannate and other copper compounds in which the ratio of copper to tin is maintained at a value to produce a desired plate and which, duringcontinuous operation, is regenerated by addition of copper and tin in the same ratio. At the same time the addition of any deleterious anions or cations to the bath is minimized or avoided entirely. A deleterious ion is defined as one which tends to build up continuously, thereby throwing the bath composition permanently out of adjustment. The fact that the bath can be formed and regenerated while avoiding high concentrations therein of anions and cations which are deleterious as before defined constitutes a feature of this invention which, so far as is known, has not hitherto been used in plating copper-tin alloys.

In the formation and regeneration of the bath, copper stannate is employed as one of the principal ingredients. It possesses a distinct advantage in the fact that both its anion and cation are useful as sources of plating metal. It is preferred to employ copper stannate as the sole source of the tin and either as the sole or the partial source of the copper. To increase the ratio of total copper to tin to any desired value over that present in copper stannate itself, there may be added to the copper stannate one or more copper salts in which copper is the cation and the anions of which do not adversely aiiect the bath. The mixture so obtained is dissolved, diluted with water, and employed as the bath solution. The copper salts mentioned may include materials like CuCOz, CuCn and Cu(OH)2, for example, and these may be added, alone or in combination, in solid form to the copper stannate in the desired amounts. It will be appreciated that there may be plated a range of copper-tin alloys from the corresponding baths in which the copper-tin ratio varies from that shown by copper stannate to any desired ratio of higher copper content.

The mixture described above, hereinafter referred to as the copper-tin mixture, may be prepared by simply mixing the dry salts, as described, or may be precipitated from solution by adding, for example, copper sulfate to a solution containing an alkali metal stannate and one or more soluble salts whose anions will form a precipitate with the copper of the copper sulfate and which, also, will not prove detrimental to the plating bath. Examples of such soluble salts re the alkali metal hydroxides, and carbonates, the sodium and potassium compounds being especially suitable. This method has particular advantages arising from the fact that copper sulfate is relatively inexpensive compared with other copper pig tin. Besides copper sulfate, other copper salts such as copper chloride and copper nitrate may be used.

Regarding the copper-tin mixture obtained by either of the above-mentioned methods, it can be seen that any desired copper-tin ratio is obtainable by varying the amounts of copper salts (exclusive of copper stannate) employed to form the mixture.

The plating bath is then made by dissolving the foregoing mixture in an aqueous solution of alkali metal cyanide and an alkali metal hydroxide. The resulting solution is diluted to a desired concentration and is then ready to be electrolyzed. Regeneration of this bath is accomplished simply by adding from time to time a quantity of the copper-tin mixture along with a small amount of alkali metal cyanide to dissolve it, solution of the mixture being preferably effected outside of the bath.

The non-productive or non-metal-producing ions (those exclusive of the copper and stannate ions) introduced to the bath may include, for example, when the alkali metal employed is sodium, those contained in NaCN and NaOH, addedin the formation of the bath, and those in NaCN, added in the regeneration thereof. The ions added via the NaOH are limited to the amount of this compound necessary to form the 7 bath and do not interfere with practical operation of the same. The ions added via the NaCN tend to build up in concentration during continuous operation, since NaCN is employed to regenerate, as well as to form, the bath, but the amount of NaCN introduced to regenerate is comparatively small and does not impede practical operation. Moreover, a certain amount of cyanide ion is lost by anodic action, when insoluble anodes are used, and this tends to oiTset the build up in concentration. The total amount of NaCN present, however, may be kept at a constant minimum by taking a small portion of the bath solution to dissolve the copper-tin mixture employed to regenerate the bath. This is permissible since the bath solution contains NaCN, which is a solvent for the copper-tin mixture, and since, furthermore, the copper-tin ratio in the bath is not changed either by removing bath solution, or by adding dissolved regenerating mixture as described above.

The non-productive ions introduced via the copper-tin mixture may include, for example, carbonate, cyanide, or hydroxide or all three,

, although it is preferred to limit the hydroxide and cyanide concentrations. A certain amount of hydroxide ion from this source may be tolerated, this amount to be measured, in a particular instance, by the amount lost in carbonation, i. e.,

by reaction with atmospheric carbon dioxide, and drag-out. The cyanide and hydroxyl ions are necessary constituents of the bath. The cyanide acts as a solvent for the cop-per ion and regulates the cathode efliciency of the deposition of the copper. The hydroxyl ion performs the same two functions with regard to the tin.

The following examples are submitted to illustrate the invention:

Example 1 Method of making a solid copper-tin mixture in which the copper exceeds the tin: The follownate may be employed with CuCO3 alone, or with Cu(OH) 2 alone, and that other Cu-Sn ratios may be similarly obtained.

Example 2 Method of making a mixed copper-tin precipitate in which the copper-tin ratio is increased ing compounds in the indicated amounts were mixed:

No. Compound Weight in grams gggg Copper stannate 230 (1 mol) 64:118 C11 3 124 (1 mol) 1282118 3.. C I1(OH) z 76 (.78 11101) 2 177:118

1 Sum of Nos. 1 and 2. 2 Sum of Nos. 1, 2 and 3.

The ratio of 1771118 is substantially a 60:40 ratio,

is to be understood, of course, that copper stanover that present in copper stannate but in which tin exceeds copper: Six liters of copper sulfate solution containing 2.5 kg. CuSO4.5H2O were added with stirring to 8 liters of a solution containing gms./l. tin as sodium stannate and gms./l. total N aOH. There was obtained a mixed precipitate of copper stannate, CuCOs, and Cu(OH)2 having a copper-tin ratio of about 45:53. The precipitate was washed a few times by decantation and sucked dry on a filter. It can be used in this air-dried condition or it may be dried at a moderate temperature such as 105 C. in an oven. By proceeding in a similar way there may be obtained precipitates having lower and higher copper-tin ratios, including precipitates in which the copper exceeds the tin.

Example 3 Method offorming and regenerating the plating bath solution: gms. of a copper-tin mixture obtained according to Example 2 were dissolved in 30 gms..NaOH and an amount of NaCN sufficient to dissolve it, and the solution made up to 1 liter. This was then electrolyzed, using insoluble anodes of iron, 9. current density of 48 amps/sq. ft., and bath temperatures .of 70 to 85 C. During the electrolysis, the bath was regenerated from time to time by additions of the copper-tin mixture along with NaCN until at the end of the plating run, 90 gms. of mixture and 18 gms. of cyanide had been added. There were plated out 89 gms. of metaLwhich represents all the metal originally present in the bath plus a fair proportion of that added during the run. The plate was of satisfactory characteristics.

Any suitable type of insoluble anode may be used, such as, for example, those of iron or nickel. V

The current density may be varied widely and will, of course, be correlated with thebath composition and temperature to obtain efiicient plating.

As will be understood, the tin content of the copper-tin mixture may be increased when necessary, as by adding hydrous tin oxide to the mixture. This procedure may be used in plating alloys having a high tin content.

It is thus apparent that a copper-tin alloy plating bath can be kept operating simply, by periodically replenishing it with additions of copper and tin in the proper ratio, as illustrated in the foregoing examples. The simplicity of control and flexibility of the present bath, as compared with the old soluble-anode baths, will also be apparent.

The use of solid copper-tin mixtures or mixed precipitates as a regenerating material is not confined to. baths originally formed from such mixtures but extends to other copper-tin plating baths containing. copper andtin compounds in solution as previously pointed out. With these baths, as well as with the type of bath described in Example 3, the herein-described method of regeneration represents an accurate method of replenishing such baths and eliminates the difficulties encountered when using soluble anodes.

It is not necessary that the anodes becompletely insoluble. For example, copper anodes may be used, and reliance placed upon the cop--.

per stannate to supply some of the copper content and all of the tin content to the bath; or conversely, tin anodes could be employed and the copper content gained from the copper stannate. Thus the problems involved in controlling alloy anodes or anodes of two difierent metals would be obviated.

In the light of the foregoing description, the following is claimed:

1. Method of electroplating a copper-tin alloy from a plating bath with which only a single anode circuit is used, said bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate thereto, and plating said alloy from said bath.

2. Method of electroplating a copper-tin alloy from a plating bath with which only a single anode circuit is used, said bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate and copper hydroxide thereto, and plating said alloy from said bath.

3. Method of electroplating a copper-tin alloy from a plating bath with which only a single anode circuit is used, said bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate and copper carbonate thereto, and plating said alloy from said bath.

4. Method of electroplating a copper-tin alloy from a plating bath with which only a single anode circuit is used, said bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate, copper hydroxide, and copper carbonate thereto, and plating said alloy from said bath.

5. Method of electroplating a copper-tin alloy from a plating bath with which only a single anode circuit is used, said bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate thereto, and plating said alloy from said bath, said copper stannate having been formed as a precipitate by adding copper sulfate to a solution containing alkali metal stannate.

6. Method of electroplating a copper-tin alloy from a plating bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate thereto, and plating said alloy from said bath using material that is insoluble in the bath as the only anode material.

7. Method of electroplating a copper-tin alloy from a plating bath using only a single anode circuit having anodes composed of insoluble material and of copper connected in said single circuit, said bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate thereto, and plating said alloy from said bath.

8. Method of electroplating a copper-tin alloy from a plating bath using only a single anode circuit having one or more anodes composed of copper connected in said single circuit, said bath containing an aqueous solution of alkali metal cyanide and alkali metal hydroxide and having copper and tin dissolved therein, which comprises regenerating the bath by adding copper stannate thereto, and plating said alloy from said bath.

FREDERICK A. LOWENHEIM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,970,548 Batten Aug. 21, 1934 1,970,549 Batten Aug. 21, 1934 2,079,842 Cinamon May 11, 1937 2,198,365 Cinamon Apr. 23, 1940 2,216,605 Sklarew et a1. Oct. 1, 1940 2,397,522 Baier Apr. 2, 1946 OTHER REFERENCES Mellor, vol. 7, Treatise on Inorganic Chemistry, page 418 (1927). 

1. METHOD OF ELECTROPLATING A COPPER-TIN ALLOY FROM A PLATING BATH WITH WHICH ONLY A SINGLE ANODE CIRCUIT IS USED, SAID BATH CONTAINING AN AQUEOUS SOLUTION OF ALKALI METAL CYANIDE AND ALKALI METAL HYDROXIDE AND HAVING COPPER AND TIN DISSOLVED THEREIN, WHICH COMPRISES REGENERATING THE BATH BY ADDING COPPER STANNATE THERETO, AND PLATING SAID ALLOY FROM SAID BATH. 